In computing systems, such as desktop computers, portable computers, personal digital assistants (PDAs), servers, and others, storage devices are used to store data and program instructions. One type of storage device is a disk-based device, such as a magnetic disk drive (e.g., a floppy disk drive or hard disk drive) or an optical disk drive (e.g., a CD or DVD drive). Disk-based storage devices have a rotating storage medium with a relatively large storage capacity. However, disk-based storage devices offer relatively slow read-write speeds when compared to operating speeds of other components of a computing system, such as microprocessors and other semiconductor devices.
Another type of storage device is a solid state storage device, such as a dynamic random access memory (DRAM) device, static random access memory (SRAM) device, flash memory device, and electrically erasable and programmable read-only memory (EEPROM) device. Another variant of a solid state storage device is a magnetoresistive solid state storage device, such as a magnetoresistive random access memory (MRAM) device. A typical MRAM device includes an array of memory cells (each memory cell made up of a magnetoresistive element) that are selected by word lines extending along rows of the memory cells, and bit lines extending along columns of the memory cells. In one conventional implementation, each memory cell is located at a cross point of a word line and a bit line.
Examples of magnetic memory cells include tunneling magnetoresistance (TMR) memory cells, giant magnetoresistance (GMR) memory cells, or colossal magnetoresistance (CMR) memory cells. These types of memory cells are commonly referred to as spin valve memory (SVM) cells.
An SVM cell includes two magnetic layers that are separated by a dielectric layer. The orientation of magnetization of one of the magnetic layers can be altered, while the orientation of magnetization of the other magnetic layer is fixed or “pinned” in a particular orientation. The magnetic layer having alterable magnetization is typically referred to as a “data storage layer” or “sense magnetic layer,” while the magnetic layer that is pinned is typically referred to as a “reference layer” or a “pinned magnetic layer.” The dielectric layer is an insulating tunnel barrier sandwiched between the magnetic layers.
The SVM cell exhibits tunneling magnetoresistance (TMR), giant magnetoresistance (GMR), or colossal magnetoresistance (CMR), in the presence of the magnetic field provided by the magnetic layers. Relative orientation and magnitude of spin polarization of the magnetic layers determine the resistance of the SVM cell. Generally, resistance of the SVM cell is a first value R if the magnetic layers have a parallel magnetization orientation, and the resistance is increased to a second value R+ΔR if the magnetization orientation is changed from parallel to anti-parallel.
The two magnetization orientations, parallel and anti-parallel, of an SVM cell represent different data states, such as logical “0” and “1.” The orientation may be changed from parallel to anti-parallel, or vice-versa, by applying the proper magnetic field to the SVM cell. Typically, the orientation of magnetization of a storage cell is a stable orientation that does not change until a magnetic field is applied to change the orientation of magnetization. Therefore, an MRAM device is able to provide non-volatile storage. The magnetization orientation of a selected memory cell is detected by measuring the resistance of the SVM cell associated with the selected memory cell.
Typically, an MRAM device includes sense amplifiers to detect states of memory cells. Some conventional sense amplifiers include digital comparators that perform comparison of states of a memory cell during a read operation. Analog-to-digital conversion is performed on signals received from the storage cell, with the outputs of the analog-to-digital conversion provided to the inputs of the digital comparator. The analog-to-digital conversion process performed at the inputs of the comparator is relatively time consuming, which may result in reduced read access speeds in MRAM devices that employ sense amplifiers with digital comparators.